The present invention relates to a key input circuit for use in a keyboard, and particularly, to a key input circuit for outputting signals as the keys of a keyboard are operated.
Keyboards used as input devices for information-processing apparatuses, such as computers, are known. Each of these keyboards features a key input circuit which has a number of keys arranged substantially in a given plane, and which are assigned to characters such as letters, numerals, and symbols. As the keys are selectively operated, the key input circuit generates and supplies coded signals, corresponding to the characters, to the information-processing apparatus. This type of input circuit generally has key switches which have mechanical contacts.
A key input circuit of this type is known which comprises 16 drive lines, M sense lines, and key switches. The drive lines are parallel to each other, and extend at right angles to the sense lines, which are also parallel to each other. The key switches are provided at the intersections of the drive lines and the sense lines. Hence, this key input circuit is a matrix circuit. Each drive line is connected, at one end, to a power supply by a resistor, and at the other end, to a drive circuit. Each sense line is connected, at one end, to the power supply by a resistor, and at the other end, to an input buffer.
When two or more adjacent key switches are depressed simultaneously, a few other keys located close to these depressed ones go into a turned-on condition, thus generating character code signals, or else character code signals corresponding to none of the key switches are output from the key input circuit. The code signals generated by the undepressed key switches, and those corresponding to none of the key switches, are called "ghost character signals."
In order to prevent the generation of such ghost character signals, it has been proposed that a diode be coupled to the contact of each key switch. If this proposal is adopted, the key input circuit would require as many diodes as there are key switches, with the result that the switch section and circuit board of the key input circuit would inevitably become complex.
Let us think of an equivalent circuit of each key switch used in this key input circuit. In the equivalent circuit, a drive line coupled to the drive circuit is connected to the cathode of the diode. The anode of the diode is coupled to one end of the key switch. The other end of the key switch is connected by a resistor to the power supply. A sense line coupled to the input buffer is connected to the connection point of the key switch and the resistor.
With this key input circuit, in order to convert the contact signal of each key switch to an electrical signal, the input buffer must recognize that a signal at the logic "0" level is on the sense line coupled to this key switch, when the key switch is turned on. Let us assume that the drive circuit is SN74LS145, and the input buffer is SN74HC541. In this case, maximum voltage V.sub.iL (max) at which the input buffer can recognize logic "0" is 1.2 volts. Hence, unless the potential at the connection point of the key switch and the resistor is 1.2 volts or less, the input buffer cannot recognize logic "0", even if the key switch is turned on.
When the drive circuit sets the drive line coupled to the key switch to the logic level "0", the potential at the connection point of the drive circuit and the diode is 0.25 volts, since the reference voltage V.sub.OL for the logic "0" of drive circuit is 0.25 volts. When the forward voltage V.sub.F of the diode is 0.7 volts, the potential applied between the connection point of the drive circuit and the diode and the connection point of the key switch and the diode is 0.7 volts. Thus, the potential applied between the key switch and the diode can be given as: EQU 0.25+0.7=0.95 (V)
Therefore, the margin between this potential and the sense potential of the key switch, which is 1.2 volts, is: EQU 1.2-0.95=0.25 (V)
Due to this relatively small margin, the potential between the key switch and the diode may increase above 1.2 volts under the influence of conditional changes, such as a higher temperature, resulting in the changes in V.sub.OL and V.sub.F, and due to the fact that V.sub.OL and V.sub.F are different from the design values. Therefore, the key input circuit of this structure is not sufficiently reliable.
In order that the input buffer correctly recognize logic "0" when any key switch is turned on, the maximum voltage drop, V.sub.RS (max), resulting from the contact resistance Rs of the key switch, must be equal to or less than: EQU 1.2-0.95=0.25 (V)
When the key input circuit is manufactured, each key switch has such contact resistance that maximum voltage drop V.sub.RS (max) is less than 0.25 volts. As the key switch is repeatedly turned on and off, however, its contact resistance gradually increases. The voltage drop at the key switch will rise above 0.25 volts after a comparatively short period of use (e.g., after the switch has been turned on and off about 5,000,000 times). If this is the case, the input buffer can no longer recognize logic "0" when the key switch is turned on. At this point, the key switch is considered to have reached the end of its useful life. Thus, the key switches of the key input circuit have a relatively short useful life.
The maximum value, Rs(max), of contact resistance Rs of the key switch can be given as follows: ##EQU1## Where, R is the resistance value of the resistor connected between the key switch and the power supply. Thus, EQU Rs(max)=0.65R(.OMEGA.)
Consequently, key switches having a contact resistance over 0.065R cannot be used in the conventional key input circuit of the structure described above.